HeartGold also comes with the Pokewalker, which is essentially an updated Pocket Pikachu. You can upload a Pokemon to the Pokewalker and take them with you. The walker serves as a pedometer and the Pokemon gains experience as the player takes more steps. It's a great item, though somewhat gimmicky, and gives the game an extra incentive to purchase it. I have to admit that I wasn't completely happy with it though, the infrared connection to the game seemed incredibly weak, and it didn't add a great deal to the game overall. I could do without it, but I wouldn't consider it a complete waste. Maybe this will help battle childhood obesity? I can easily see some kid running around outside with the Pokewalker just to get experience for their Pokemon.
While there has been some criticism out there that HeartGold and SoulSilver are just lazy remakes of Gold and Silver, I think it was needed due to the battery issues in the older games, particularly since Gold and Silver are held in such high regard by fans. HeartGold is the ultimate nostalgia game for Pokemon fans, and perfect for long-time fans and newer, younger fans.
Pokemon HeartGold is rated E (Everyone) by the ESRB for Mild Cartoon Violence.